A simplified model is proposed to determine the thermodynamic constants of the transport reaction in a close-spaced configuration, even when the reacting species are unknown. For example, the enthalpy variation, the activation energies of condensation, and sublimation in the water vapor transport of CdTe are determined (AHeq. = 22, Qcond. ----9.2, and QsubL = 31.2 in kcal/ mole-D. The model is useful for interpreting the experimental results in epitaxial processes. In the low pressure iodine transport of silicon, it is shown that the growth rate is not a diffusion controlled process; from the energy AHeq : --16.5 kcal/mole -1 for the reaction Si + 2I ~--SiI2, we found Qcond. "----8 and Qsubl. : --24.5 (kcal/mole-D. An important result of the analysis is that determination of enthalpy variation cannot be obtained only from the slope of the growth rate-temperature dependence.The use of a close spacing in chemical transport systems for growing epitaxial layers of pure or compound semiconductors has been emphasized by Sirtl (1), Robinson (2), and Nicoll (3). The most important features of the technique are the use of a close spacing between source and substrate (1 mm) between which is maintained a temperature gradient, and the fact that a gas is used to react with the source to form a volatile compound which is decomposed at the surface of the substrate. Generally, the transport reaction occurring in the closed spaced system is difficult to establish, and it is the aim of this paper to propose a simple theoretical model for the mass transfer even if the transporting species are unknown. Although the deposition rate is strongly sensitive to the surface preparation and the crystallographic orientation of the substrate (4, 5) as well as to the pressure, or the flow rate, of the reactive gas, the substrate and source temperatures are of major importance in the process only in so far as the single crystallinity of the layer is not concerned. The proposed model is shown to be useful for endothermic as well as exothermic process, i.e., for positive or negative heat of reaction at constant pressure.